We are currently in the middle of developing a recombinational method for testing homologies between different segments of Drosophila DNA which have been inserted into plasmids. Two different segments, incorporated into two different plasmids, are introduced into the same bacterial cell. If they share nucleotide sequences, recombination might be expected between them, thus creating a single chimeric plasmid which we can score. At this point, no recombination has been detected. If such a scheme could be perfected, it should be possible to map the entire Drosophila genome by standard microbiological techniques. BIBLIOGRAPHIC REFERENCES: D.H. Hamer and C.A. Thomas, Jr.: The Cleavage of Drosophila melanogaster DNA by Restriction Endonucleases." Chromosoma 49, 243 (1975). M.D. Bick and C.A. Thomas, Jr.: "A New Method for Mapping Nucleic Acid Sequence Homology by Electron Microscopy." Nuc. Acids Res. 2, 509 (1975).